1. Field of the Invention
The invention relates to a radar system which utilizes a rotating or pivotal antenna lobe and a primary radar device operating with doppler evaluation and a secondary radar device equipped with a rotating or pivotal small antenna lobe, and more particularly to such a system wherein a certain time period expires between the scanning of the recorded target by the lobe of the primary radar antenna and the recognition of the target during target evaluation in the receiver of the primary radar device.
2. Description of the Prior Art
It is common practice in secondary radar devices, within the framework of the air space control and air traffic control, to transmit interrogation signals consecutively with a fixed interrogation frequency. In the case of cooperation with primary radar devices, it is furthermore known to provide a lead time (pre-trigger time) and therefore guarantee that the responses from a transponder and the echo signals of the primary radar device are provided for processing approximately at the same time. In this connection see the book "Secondary Radar," Peter Honold, Siemens Aktiengesellschaft, Berlin and Munich, (1971) Pages 35-36. The antennae of both systems are always equally directioned and the interrogation signals are transmitted independently of whether targets have been scanned by the primary radar. Therefore, the work load in the radio space is often undesirably increased, and, due to the different ranges of primary radar and secondary radar, reply signals are also triggered by targets which are not of interest. This causes an unnecessary load on the transponders and the secondary radar receivers.
The German Letters Pat. No. 1,263,872, discloses an air control system wherein a certain time section is assigned to each airplane in such a way that each airplane receives interrogation signals only in the time section associated therewith, and transmits reply signals during this time. Such methods result in an essential decrease in the number of reply signals, since answers are not triggered continuously, but only on a section basis. However, such systems have the drawback that clocks of extreme accuracy are required for controlling the transponders. The number of the interrogation signals and thus the radio-space load remains the same.